So-called bottom-up fabrication methods aim to assemble and integrate molecular
components exhibiting specific functions into electronic devices that are
orders of magnitude smaller than can be fabricated by lithographic techniques.
Fundamental to the success of the bottom-up approach is the ability to control
electron transport across molecular components. Organic molecules containing
redox centres—chemical species whose oxidation number, and hence electronic
structure, can be changed reversibly—support resonant tunnelling1, 2 and display promising functional behaviour when sandwiched as
molecular layers between electrical contacts3, 4, but their
integration into more complex assemblies remains challenging. For this reason,
functionalized metal nanoparticles have attracted much interest5, 6, 7:
they exhibit single-electron characteristics8, 9, 10 (such as
quantized capacitance charging) and can be organized11, 12, 13
through simple self-assembly methods into well ordered structures, with the
nanoparticles at controlled locations. Here we report scanning tunnelling
microscopy measurements showing that organic molecules containing redox centres
can be used to attach metal nanoparticles to electrode surfaces and so control
the electron transport between them. Our system consists of gold nanoclusters
a few nanometres across and functionalized with polymethylene chains that
carry a central, reversibly reducible bipyridinium moiety14, 15.
We expect that the ability to electronically contact metal nanoparticles via
redox-active molecules, and to alter profoundly their tunnelling properties
by charge injection into these molecules, can form the basis for a range of
nanoscale electronic switches.